Heavy-duty, lean-burn, natural-gas engines have found many applications in the stationary ergine marketplace because of their ability to operate more efficiently than their stoichiometric counterparts. However, these engines produce a significant amount of nitrogen oxide (NO.sub.x) emissions. Presently, for these engines, there are no cost effective methods of exhaust gas after-treatment to reduce NO.sub.x emissions to levels mandated by various legislative authorities.
In lean-burn engines, oxidation catalysts have been used to oxidize unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. However, NO.sub.x is not affected and is typically controlled by operating the engine as lean as possible to take advantage of the heat capacity of the excess air. The excess air in the air/fuel charge functions as a heat sink to reduce in-cylinder gas temperatures and minimize the rate of NO.sub.x formation.
In stoichiometric engines, three-way catalysts are used to control the emissions of NO.sub.x, unburned HCs, and CO simultaneously. These three-way catalysts provide better exhaust gas after-treatment than is available in the case of lean-burn engines.
The trade-off between stoichiometric operation and lean-burn operation is economic, manifesting itself in the thermal efficiency/NO.sub.x relationship. Engine operators incur an economic penalty by operating stoichiometric engines for legislative compliance because stoichiometric engines are currently less efficient than comparable lean-burn engines.
A new method of controlling NO.sub.x emissions in lean-burn engines involves the use of syngas. Tests have been performed on a multi-cylinder engine where bottled blends of selected syngas components were used to simulate natural gas/syngas mixtures. The tests were conducted over a range of equivalence ratios and syngas fractions where it was shown that NO.sub.x could be held to less than 30 ppm (.about.0.3 g/bhp hr) at a BTE (brake thermal efficiency) of 39 percent. Similar work has been conducted on gasoline engines with air-reformed gasoline. In those tests it was shown that an engine could operate at leaner equivalence ratios than on pure gasoline. In addition, NO.sub.x decreased because in-cylinder temperatures were reduced due to the added heat capacity of the excess air.
In stoichiometric engines, exhaust gas recirculation (EGR) has been used to reduce NO.sub.x emissions as well as to increase efficiency by allowing the engine to operate with increased boost pressures and compression ratios. However, the maximum EGR dilution level is quickly reached, and excess EGR causes misfire. The typical engine EGR tolerance is about 25% of the total fuel mix.